· Emitter current IE flows in the input circuit. Collector current IC flows in output circuit.

· The ratio of collector current IC, to emitter current IE, is called the

Current amplification factor (α).

· If there is no input ac signal, then the ratio of IC to IE is called dc alpha (αdc).

· ac alpha refers to the ratio of change in IC to change in IE.

· The higher the value of α, better the transistor.α can be increased by making base thin and lightly doped.

Characteristics of CB configuration

The performance of transistors, when connected in a circuit, may be determined from their characteristic curves that relate different d.c. currents and voltages of a transistor.Such curves are known as Static characteristic curves.

Input Characteristics

· The curve drawn between Emitter current and Emitter – Base voltage for a given value of collector-Base voltage is known as input Characteristics.

· For a given value of VCB,the curve is just like a forward-biased PN junction.

· With an increase in the value of VCB,it conducts better. This is because of the effect called early effect or Base width modulation.

Output Characteristics

· The curve drawn between Collector current and Collector – Base voltage for a given value of emitter current is known as output Characteristics.

· The collector current varies with VCB for very low voltage but transistor is never operated in this region.

· Base current IB flows in the input circuit. Collector current IC flows in output circuit.

· CE is commonly used because its current, voltage and power gains are quite high and output to input impedance ratio is moderate.

· The rate of change in collector current IC, to change in emitter current IE, is called amplification factor (β).

Input Characteristics

· The curve drawn between Base current and Base – Emitter voltage for a given value of collector-emitter voltage is known as input Characteristics.

· For a given value of VEC,the curve is just like a forward-biased PN junction diode.

· Input resistance is larger in CE configuration than in CB configuration. This is because the input current IB increases less rapidly with increase in VBE .

· An increment in the value of VCE, causes the input current IB to be lower for a given level of VBE . This is because of the effect called early effect.

Output Characteristics

· The curve drawn between Collector current IC and Collector – emitter voltage VCE for a given value of base current IB is known as output Characteristics.

· Output characteristics in CE configuration have some slope while CB configuration has almost horizontal characteristics. This indicates that output resistance in case of CE configuration is less than that in CB configuration.

· The Collector forms the terminal common to both the input and output.

· Base current flows in the input circuit. Emitter current flows in output circuit.

· With base current IB equal to VCO, the emitter current IE is zero, so no current flows in the load resistor RL.

· With increases in input current IB, the transistor passes through the active region and finally reaches saturation.

Input Characteristics

· To determine the input Characteristics, VEC is kept at a suitable fixed value.

· The base-collector voltage VBc is increased in equal steps and the corresponding increase in IB is noted.

· This is repeated for different values of VEC.

Breakdown in Transistors

Avalanche Multiplication

Reach-Through (or) Punch through

Avalanche Multiplication

· The maximum reverse bias voltage which can be applied before breakdown between collector and base terminals of the transistor under the condition that the emitter is open-circuited.

· It is represented by the symbol BVCBO (for CB configuration).

· This breakdown voltage is a characteristic of the transistor alone.

· Breakdown occurs because of the avalanche multiplication of current ICO that crosses the collector junction.

· As a result of this multiplication, the current becomes MICO in which M is the factor by which the original current ICO is multiplied by the avalanche effect.

· At a high voltage BVCBO, the multiplication factor M becomes infinite and the region of breakdown is then attained.

· The current increases abruptly and large changes in current accompanies small changes in voltage.

Reach-Through (or) Punch through

· It results from Early effect (i.e.) as a result of increase in VCB and as the doping of the base is substantially smaller than that of the collector and the penetration of the transition region into the base is larger than into the collector

· Since the base is very thin, the transition region spreads completely across the base to reach the emitter junction.

· At this point, normal transistor action ceases and the emitter and collector are effectively shorted.

· Hence, a large current flows from the emitter to collector. This is called Reach-through.

Field Effect Transistor (FET)

FET is a semiconductor device which depends for its operation on the control of current by an electric field.

The output characteristics of FET are controlled by Input voltage and not by the Input current.

So, it is also known as voltage-controlled device.

Features of FET

The FET has several advantages over the conventional transistor.

Its operation depends upon the flow of majority carrier only. So, it is called as Unipolar device.

It is relatively immune to radiation.

It exhibits a high input resistance, typically many mega ohms.

It is less noisy than a tube of a Bipolar Transistor.

It exhibits no offset voltage at zero Drain current.

It has thermal stability.

Types of FET

Junction Field Effect Transistor (JFET)

Metal Oxide Field Effect Transistor (MOSFET) (or)

Insulated Gate Field Effect Transistor (IGFET)

Construction of JFET

JFET is a three terminal semiconductor device in which current conduction is by one type of carrier either Electrons or holes.

The JFET consists of a P-type or N-type silicon bar.

The bar is made up of N-type material which is known as N-channel JFET and if the bar is made up of P-type material, it is known as P channel JFET.

The current in FET is carried by the majority carriers.

One end of the channel is called the source and the other is called the drain.

Operation of JFET

FET works under the three conditions.

When VGG applied and VDD=0

When VDS applied and VGG=0

When VDD applied and VGG is applied.

Where,

Ø VGG – Gate supply voltage.

Ø VDS– Drain Source voltage.

Ø VDD– Drain supply voltage.

Characteristics of JFET

· A family of curves that relate the current and voltage are known as characteristics curve.

· There are the two important characteristics of a JFET.

Ø Transfer characteristics

Ø Drain characteristics

Characteristics Parameters of JFET

The parameters of JFET are

Transconductance

Drain resistance

Drain conductance

Amplification factor

Metal Oxide Semiconductor FET (MOSFET)

MOSFET is a three terminal device. Those terminals are source, gate and drain.

The gate of a MOSFET is insulated from the channel.

Because of this, the MOSFET is also known as an IGFET (Insulated gate FET).

The MOSFET is a second category of FET.

The MOSFET differs from the JFET is that it has no pn junction structure; instead the gate of the MOSFET is insulated from the channel by a silicon dioxide layer.